r - Asymmetric expansion of ggplot axis limits

Question

How do you adjust the expansion of limits asymmetrically in ggplot? For example,

library(ggplot2)

ggplot(mtcars) + 
  geom_bar(aes(x = cyl), width = 1)

I would like the bottom of the bars flush with the bottom of the panel background, but would still like space at the top. I can achieve this with a blank annotation:

ggplot(mtcars) + 
  geom_bar(aes(x = cyl), width = 1) +
  annotate("blank", x = 4, y = 16) +
  scale_y_continuous(expand = c(0.0,0)) 

In previous versions of ggplot, however, I could use the solution provided by Rosen Matev:

library("scales")
scale_dimension.custom_expand <- function(scale, expand = ggplot2:::scale_expand(scale)) {
  expand_range(ggplot2:::scale_limits(scale), expand[[1]], expand[[2]])
}

scale_y_continuous <- function(...) {
  s <- ggplot2::scale_y_continuous(...)
  class(s) <- c('custom_expand', class(s))
  s
}

and then use scale_y_continuous(expand = list(c(0,0.1), c(0,0))) which would add a consistently addition to the top of the chart. In the current version, however, I get an error

ggplot(mtcars) + 
  geom_bar(aes(x = cyl), width = 1) +
  scale_y_continuous(expand = list(c(0,0.1), c(0,0)))

# Error in diff(range) * mul : non-numeric argument to binary operator

Is there an effective solution for ggplot2 2.0?

A solution should include the ability to work flexibly with facets, and free_xy scale options. For example,

ggplot(mtcars) + 
  geom_bar(aes(x = cyl, fill = factor(vs)), width = 1) + 
  facet_grid(vs ~ ., scales = "free_y")

A solution should provide something like:

ggplot(mtcars) + 
  geom_bar(aes(x = cyl, fill = factor(vs)), width = 1) + 
  facet_grid(vs ~ ., scales = "free_y") + 
  scale_y_continuous(expand = c(0,0)) + 
  geom_blank(data = data.frame(cyl = c(5,5), y = c(12, 16), vs = c(1,0)), aes(x = cyl, y = y))

Answer 1

ggplot2 v3.0.0 released in July 2018 has expand_scale() option (w/ mult argument) to achieve OP's goal.

Edit: expand_scale() will be deprecated in the future release in favor of expansion(). See News for more information.

library(ggplot2)

### ggplot <= 3.2.1
ggplot(mtcars) + 
  geom_bar(aes(x = cyl, fill = factor(vs)), width = 1) + 
  facet_grid(vs ~ ., scales = "free_y") + 
  scale_y_continuous(expand = expand_scale(mult = c(0, .2))) 

### ggplot >= 3.2.1.9000
ggplot(mtcars) + 
  geom_bar(aes(x = cyl, fill = factor(vs)), width = 1) + 
  facet_grid(vs ~ ., scales = "free_y") + 
  scale_y_continuous(expand = expansion(mult = c(0, .2)))